How/where can i download old Apple keynotes ?

All of you responding thus far have been misinterpreting the original poster's inquiry. They are not asking about the presentation app Keynote. They are asking about the Apple WWDC Keynote Presentations.

As to the answer to the question originally asked, this is the App Store community forum. So I can see how the others got confused. Next time ask your question in the appropriate forum.

You can find old WWDC keynotes in iTunes' Podcasts section. Just search for "Apple Keynotes" and you'll find them no problem.

The SD versions go back to 2007, the HD ones only as far back as 2011.

If you want to keep any of the presentation videos, it might be a good idea to click the little cloud icon to download the podcast episode from the server.

Only the newest version is available with Yosemite.

Some users whose Macs are relegated to an older version of OS X because of hardware restrictions can sometimes get an older version of apps that will run on their Mac, but not someone with a Mac running Yosemite. Apple expects that folks keep backups of older versions they may wish to use again.

Apple don't let download old keynotes but you could get it from a third party website.

You cannot legally get Mac App Store versions of Apple apps from 3rd party sites. They would be pirated.